Immediate impact of the 2021 harmful algal bloom in southeast Hokkaido on the rocky intertidal community and its spatial variation

There has been a limited number of studies on the effects of harmful algal blooms (HABs) on natural rocky intertidal ecosystems. From mid-September to early November 2021, an unprecedented HAB caused by Karenia selliformis hit the Pacific coast of southeast Hokkaido, Japan, for the first time, causing massive mortalities among marine organisms. To clarify the immediate impacts of the HAB on abundance of 10 rocky intertidal species in four functional groups (macroalgae, sessile invertebrates, molluscan grazers, and molluscan carnivores), we focused on two questions. 1) How did the HAB affect the abundance of each species differently at the regional scale? 2) How did the impacts of the HAB on the abundance of each functional groups vary spatially, and was the spatial variation of the HAB impacts related to the spatial distribution of cell density of HAB species? To study these issues, we compared census data for 17 years before the HAB and within one month after it for five shores on the southeast coast of Hokkaido. Results showed that two macroalgae species and all three molluscan grazer species declined significantly after the HAB. Moreover, the decrease of molluscan grazers was significantly correlated with cell density. These results suggest that the impacts of the HAB in southeast Hokkaido on abundance of rocky intertidal organisms are highly variable depending on species and locality, presumably because of differences in species-specific tolerances to HAB toxins and spatial variation in the density of the HAB organisms.